Recycling: A process as circular as the earth?

Written by: RWW | Published:

Closed loop recycling and the circular economy are buzzwords in today’s waste management sector, however, what about the way minerals have been continuously recycled over the centuries? Dr Tim Johnson, technical director, Tetronics looks at the role thermal treatment plays in helping to recycle essential minerals.

Volcanoes are one of the greatest natural wonders of the world. They open a window into a weird subterranean landscape of immense pressures and temperatures, alternately fascinating and frightening, responsible for the downfall of civilisations, the preservation of ancient towns for future generations, the alteration of global weather patterns and (rather more prosaically) the disruption of thousands of air travellers a few years ago.

Interestingly, volcanoes are also the ultimate origin of the igneous and metamorphic rocks that make up nearly all the top surface of the earth and are therefore the source of pretty much all the minerals upon which our modern lifestyles depend.

In our digitally interconnected lives it is easy to forget just how reliant we are on minerals for the things we take for granted.

Almost all manufactured objects contain materials that had to be dug out of the ground at some point, then processed and transported, all using equipment made from yet more minerals.

Web of connections

Indeed, every part of the journey from mine to factory to ultimate destination is dependent on other objects that owe their existence to the rocks beneath our feet and in turn to the volcanoes that created them. When you start to see this extraordinary web of connections that stretch right around the world, you realise just how much we truly are interconnected, not only with each other, but also with the earth itself and the geological forces that shaped the world around us.

When at last these objects and the materials they contain reach the end of their useful life they become a waste.

Having been lying quietly for millions of years, these materials have a brief moment of use and are then discarded.

Thankfully, these days increasing amounts of waste are being recycled.

In part, this is a result of changes to government policies and legislation at both the UK and EU levels.

However, it is also because there is an increasing awareness in society of the environmental and economic benefits of recycling and a growing sense of social responsibility, which are working together to change our behaviour.

One only has to think of the dazzling array of coloured bins, boxes and bags for our domestic rubbish and the positive impact this has had on local authority recycling rates in the last few years to see this sense of collective responsibility in action.

These factors have also led to the development of all kinds of new environmental technologies that have made it easier to recycle more of our wastes.

However, despite all this there will always be a significant portion of our waste that cannot be recycled (at least not economically) and increasingly this is being diverted from landfill to various thermal treatments.

Public acceptance

For me, one of the most striking changes in the UK waste scene over my lifetime has been the growing public acceptance (albeit very grudgingly) of the need for more waste incinerators and thermal treatment plants.

I find the number of these facilities in various stages of build and planning truly remarkable, given the degree of public animosity to these technologies a few decades ago, and this must surely signal a growing maturity in our relationship with the waste we produce.

This can only be a good thing, but personally I look forward to the day when communities take this one step further and start to take care of their wastes rather than just hoping someone else will make the problem go away, because they understand more fully the extent and value of the resources contained in them.

That would really produce a revolution in recycling and re-use rates.

Returning to our waste stream, we can continue to follow the trail of the minerals and for the most part they end up in the ash.

Incinerator bottom ash (IBA) is relatively benign and after much testing and evaluation is now commonly approved for use as an aggregate.

Conversely, many of the more hazardous species from the waste, such as heavy metals (lead, arsenic, cadmium, mercury, etc), chlorine, sulphur, dioxins, furans, etc, tend to concentrate into the fly ash, which means the resulting air pollution control residues (APCR) are classified as hazardous wastes and need to be treated accordingly.

Fastest growing hazardous waste stream in the UK

APCR is currently being generated in the UK at a rate of around 200,000 tonnes per year; looking at the number of new waste to energy plants likely to come on stream, this will more than double in the next five years to around 500,000 tonnes per year and many in the UK waste industry believe this is likely to be an underestimate. This makes it the fastest growing hazardous waste stream in the UK today. There is are a number of potential treatment options for APCR and one of the key methods is provided by Tetronics’ DC plasma arc technology.

This technique has been used in a wide range of applications for many years and is ideally suited not only to addressing the hazardous nature of wastes, but also to the recovery of useful materials from them.

In a typical plasma waste treatment system, the APCR is blended with fluxing additions (often other wastes) to lower the melting point of the slag before being fed into the plasma furnace (see Figure 1), which is a refractory lined vessel similar in construction to an electric arc furnace.

The plasma arc runs from a graphite electrode to the surface of a molten bath of slag, which floats on a layer of liquid metal in the bottom of the plasma furnace.

The waste material feed melts into this molten bath, while the more volatile species vaporise and pass through a standard gas cleaning system for compliant discharge to atmosphere.

APCR from domestic waste contains substantial levels of chlorine (often 15 to 20wt% of the ash) and this is recovered as a clean, concentrated hydrochloric acid in the gas cleaning system for resale as a pickling acid and similar industrial uses.


The overwhelming majority of the very fine ash that goes into the plasma furnace emerges as a slag-based product, known as Plasmarok (pictured above).

This glassy material has been approved by the UK Environment Agency for sale as a secondary aggregate because of its inert low-leaching characteristics and its excellent mechanical strength, which is better than many natural equivalents such as granite and basalt.

Just as important, it is a material that can be recycled after its next application and beyond using standard techniques without raising the spectre of future environmental problems.

The DC plasma arc treatment of APCR has been the subject of extensive development by Tetronics and its partners over the past few years.

This has led to significant reductions in the flux addition required, from typically over 30% of the ash weight to around 10%, and in furnace operating temperatures, which have reduced from c.1500°C to c.1200°C.

Together these changes have reduced the cost of electricity and fluxes, leading to substantial improvements in process economics.

Work is also continuing to explore higher-value applications for the Plasmarok and to improve the quantity and quality of the recovered acid, which are bringing significant improvements in process income.

And what of our intrepid minerals in all this?

Well, having made their journey from volcano to material to object to waste, they can now go back through the man-made volcano that is a DC plasma furnace to become a useful material that will be recycled again in its turn.

Truly, this is a waste treatment solution that is as circular as the earth itself.

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